Located in the midst of the northern Andes, the Mercaderes-Rio Mayo xenolith occurrence presents a unique insight into the crustal and mantle make up of the Andes in this area. The xenoliths are found in the Granatifera Tuff, which is a small tuff cone or tuff ring exposure. The Mercaderes-Rio Mayo xenolith suite contains heterogeneous assemblages of mantle-derived, crustal and igneous rocks, which attest to the heterogeneity of the crust and mantle in the area. The mantle at Mercaderes comprises mainly garnet peridotite at deeper levels, and shows a transition to websterites and pyroxenites towards the crust-mantle boundary. These are probably a product of intrusions of melts originated within a mantle modified by fluids released from a subducting slab. The lower crust comprises a variety of hydrated (Hbl + Bio) (e.g. hornblendites and pyribolites) and dehydrated (Cpx) (e.g. pyroxenites and granulites) rocks. These rocks represent the transition from amphibolite facies to granulite facies. Petrographical, geothermobarometric and geochemical evidence indicates that this involves the dehydration and probable melting of the hydrated rocks to produce the dehydrated rocks (restites). The protoliths for the lower crustal rocks include rocks that were tectonically accreted (lateral accretion) and rocks that were intruded into the crust (vertical accretion). The igneous rocks in Mercaderes are volcanic lamprophyric rocks and andesites, and diorites. The lamprophyric rocks are possibly the host for the mantle xenoliths, suggesting that their source lies within the mantle. The andesites show similarities with adakites, and it is likely that their magma formed with considerable crustal contribution, possibly from the dehydration of lower custal rocks. The magma sources in Mercaderes lie, therefore, within the mantle and the crust, and are possibly the result of mixing between these two. From the xenoliths from Mercaderes-Rio Mayo three processes involved in the crust and mantle formation beneath Mercaderes can be identified: Lateral tectonic accretion; intrusion of magmas into the upper mantle and lower crust and possible melting from restites. The model presented here suggests that the latter two are predominant in the deeper levels of the crust in the area.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:696496 |
| Date | January 1998 |
| Creators | Weber, Marion B. I. |
| Publisher | University of Leicester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/2381/30431 |
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